This program (http://chezjd.free.fr/Creation/logiciel.php?sign=ImBX&lang=1) will change the grey logo in the firmware, and this one (http://keakaj.com/visage.htm) will change the OS x boot screen.
Other programs are (undoubtedly) available, use at your own risk, blah blah yakkity smakkity...
Once you've sorted the case...
Other programs are (undoubtedly) available, use at your own risk, blah blah yakkity smakkity...